1. Field of the Invention
The present invention relates to an antenna apparatus mounted at a wheel rim of a tire of a vehicle, to be used as a transponder of a system for monitoring a tire pressure or the like.
2. Description of the Related Art
In recent years, systems for monitoring tire pressures have been popular. In such a system, an antenna apparatus with a pressure sensor like a quartz vibrator embedded is mounted at a rim of a wheel (a wheel rim) of a tire, so that a driver can recognize a tire pressure even while driving the vehicle. In the tire-pressure monitoring system (TPMS), the antenna apparatus used as a transponder is generally attached to the wheel rim at a side facing the space inside the tire. Such an antenna apparatus has an antenna element capable of transmitting/receiving signals to/from an external antenna provided at a vehicle body. When the antenna element receives an interrogation signal from an interrogator provided in a vehicle cabin and connected to the external antenna, tire-pressure information detected by the pressure sensor is transmitted from the antenna element to the external antenna. The communication between the antenna element and the external antenna provided at the vehicle body uses a vertically polarized wave.
The antenna apparatus used as a transponder includes the antenna element and a circuit board. The circuit board has an electronic circuit portion including a sensor circuit and a transmitting/receiving circuit. Since the antenna apparatus is rotated with the tire, a large centrifugal force or a shock may be applied to the antenna apparatus during driving of the vehicle. Thus, it is difficult to hold a required attachment strength merely by bonding the antenna apparatus on the wheel rim. To solve this, various suggestions have been made to provide attachment configurations of antenna apparatuses by using air valves provided at wheel rims (for example, see Japanese Unexamined Patent Application Publication No. 10-44726, pages 2 and 3, and FIG. 1).
However, in these known antenna apparatuses in which the antenna apparatuses are mounted at the wheel rims by using the air valves as described above, when a configuration in which a part of an air valve (e.g., a grommet) is modified to be used for the antenna apparatus is employed, the manufacturing cost may not be reduced, and the antenna apparatus may not be attached to various types of wheels later. Also, in these known antenna apparatuses mounted at the wheel rims, when a configuration in which the entire apparatus is not sufficiently reduced in height is employed, a tool or the like may come into contact with the antenna apparatus during detachment of a tire, whereby the apparatus may be damaged.
In a case where the antenna element of the antenna apparatus mounted at the wheel rim is a patch antenna, the gain thereof is necessary to be decreased to achieve reduction in height of the apparatus. When a radio wave (a linearly polarized wave) is radiated from the patch antenna of the antenna apparatus mounted at the wheel rim toward a tread of the tire, the wave may be reflected by a metal band embedded in the tread. To avoid the reflection, the patch antenna may be preferably arranged in a posture such that radial beam with a high field intensity is radiated toward an area within a sidewall of the tire. In such a case, the patch antenna is necessary to be inclined largely relative to a radial direction of a wheel to achieve the reduction in height of the apparatus, so that the radial beam with the high field intensity is radiated toward the tread of the tire. As a result, a major portion of the wave radiated from the patch antenna is reflected by the tread and the wheel rim repeatedly, and then passes through the sidewall. Due to this, an amount of vertically polarized wave component received by the external antenna provided at the vehicle body may be decreased, and the gain thereof is decreased. This may degrade communication performance.